On every single water pump, flow rate and pump head are the two most important specifications. Flow rate is how much water can be moved in time period without any restriction. This value is usually given in gallons per hour (GPH), liters per hour (LPH). A benchmarking procedure would be simply to see how much water can be moved from one tank to another in a hour. Head is how high or hard water can be pushed at full load. This value is usually given in feet or meters. A benchmarking test would be to see high the pump can push water in a thin vertical tube.
All pumps list their pump rate but not all pumps list their head. Why? Flow rate is usually a nice big number... but is an absolutely useless value without a head value. You will never use a pump in a watercooling loop without any restriction. Waterblocks and tubing bends all create restriction. For example, a pump lists its pump rate at 1000GPH but has only 1ft of head. Any type of restriction would seriously drop the actually flow rate. If there is enough restriction, the pump may not actually move any water. While the pump can push the water very fast, it simply cannot push the water hard enough.

Here is another example of an actual pump. The solid black line is for the Danger Den - DDC-12V REV2 "PLUS" while the blue line is for an imaginary pump. The DDC-12v has a flow rate of 425 LPH and a head of 3.5m. In a empty tank without restriction, it can move 425 LPH. On the other hand, it can lift a column of water 3.5m vertically but it will not be moving any water since that is full restriction. Real world performance would obviously full some where between these two points. You want your water to be moving but the loop will have restriction. The imaginary pump has a has a flow rate of 425 LPH but a head of 1m. Let say is would require .5m of head overcome all restriction and move water. The DDC-12V would still be moving water at 390 LPH but the other pump would be doing only 275 LPH. If there was .99m of restriction, the DDC-12V would be at 350 LPH and the other pump would be at near 0 LPH. Be sure to check for head and don't be fooled by high flow pumps!
All pumps list their pump rate but not all pumps list their head. Why? Flow rate is usually a nice big number... but is an absolutely useless value without a head value. You will never use a pump in a watercooling loop without any restriction. Waterblocks and tubing bends all create restriction. For example, a pump lists its pump rate at 1000GPH but has only 1ft of head. Any type of restriction would seriously drop the actually flow rate. If there is enough restriction, the pump may not actually move any water. While the pump can push the water very fast, it simply cannot push the water hard enough.

Here is another example of an actual pump. The solid black line is for the Danger Den - DDC-12V REV2 "PLUS" while the blue line is for an imaginary pump. The DDC-12v has a flow rate of 425 LPH and a head of 3.5m. In a empty tank without restriction, it can move 425 LPH. On the other hand, it can lift a column of water 3.5m vertically but it will not be moving any water since that is full restriction. Real world performance would obviously full some where between these two points. You want your water to be moving but the loop will have restriction. The imaginary pump has a has a flow rate of 425 LPH but a head of 1m. Let say is would require .5m of head overcome all restriction and move water. The DDC-12V would still be moving water at 390 LPH but the other pump would be doing only 275 LPH. If there was .99m of restriction, the DDC-12V would be at 350 LPH and the other pump would be at near 0 LPH. Be sure to check for head and don't be fooled by high flow pumps!









